JEFFERSON CITY, Mo. -- Criminal investigators at the Environmental Protection Agency are looking into a 2005 reservoir collapse that flooded a state park, injuring the park superintendent and his family, documents show.

The collapse of Ameren Corp.'s Taum Sauk reservoir in southeast Missouri washed mud, rocks and debris into the Black River, along with more than a billion gallons of water, and significantly damaged Johnson's Shut-Ins State Park. The park reopened Monday for two months while the cleanup continues.

Missouri Attorney General Jay Nixon said in May that he would not file charges based on an investigation by the Missouri State Highway Patrol. But documents obtained by The Associated Press show the EPA's Criminal Investigation Division in St. Louis received a copy of the patrol's investigation report on June 15.

Michael Burnett, the special agent in charge of the EPA's regional criminal investigation section, said Monday that he could not confirm or deny the investigation into the reservoir collapse.

According to the EPA's Web site, its criminal division "investigates the most significant and egregious violators of environmental laws."

Findings from an EPA investigation would have to go through the U.S. attorney's office for a decision on bringing charges.

AmerenUE President Tom Voss told the AP last month he was unaware of any EPA investigation or any contact by EPA officials with Ameren, AmerenUE's parent company.

An Ameren spokesman said Monday that the company had not been contacted by the EPA regarding the reservoir collapse.

"We feel another investigation is unnecessary, but if they choose to launch one, we'll cooperate fully with them as well," spokesman Mike Cleary said.

The reservoir collapse has also been investigated by the state Department of Natural Resources, the Federal Energy Regulatory Commission and the Missouri Public Service Commission, which regulates investor-owned electric companies.

Ameren has agreed to pay the energy commission a record $15 million in fines. Nixon is pursuing a lawsuit against Ameren, and the state Public Service Commission is still looking into Ameren's public safety practices.